Hi and welcome to the site. My name is***** hope I'm able to provide some helpful medical information for you today.
I'm sorry that the cause hasn't been figured out as yet. And glad that you have another ultrasound scheduled for not too far away.
Darker blood is actually more reassuring to us than bright red blood. Dark blood is usually old blood, brown and black being signs that the blood has been sitting inside the uterus
for a while before coming out. The reason this is less worrisome is that it means that you aren't having any active new bleeding going on. Just old blood still coming out.
The most common reason for abnormal bleeding pattern in your age group is hormonal but polyps or fibroids, both of which are usually not cancerous are also very common. Less common causes include liver disease, various medications and precancers or cancers. Risk of precancer or cancer is higher if a woman is obese, has a family history of uterine or colon cancer, has diabetes or has earlier in life had episodes of skipping periods.
So the bot***** *****ne is that the darkness of the blood is actually reassuring to a gynecologist. The proper next step is an ultrasound which you already have scheduled. After that depends on what the ultrasound shows but the VAST majority of the time, the cause is not cancer.
I hope this information was reassuring and helpful. Please reply if I can clarify anything or provide more details. I want to be sure you have the most helpful information for your situation.